In a conventional X-ray CT device, the scan conditions, including the tube voltage and the tube current supplied to the X-ray tube, are kept constant while scanning the same cross-section.
Further, a helical scan for taking a photograph by scanning the body being examined along a spiral path has been widely used in recent years, but the conditions of scanning in the body axis direction are maintained constant during such a scan.
Accordingly, for example, when the cross-section of the body to be examined is not a concentric circle, but is elliptical with respect to the rotating axis of a CT scanner (also called a scanner), the length of the path through the body where the X-ray passes is greatly changed in accordance with the rotating angle of the X-ray source. Therefore, a problem exists in that the amount of transmitted X-rays may be excessive or insufficient while scanning one cross-section.
Further, the X-ray absorption coefficient is greatly different in the internal organs of low density in the chest, such as the lungs, etc., and the internal organs of high density in the abdomen such as the liver, etc. Therefore, a the scan is continuously performed from the chest to the abdomen and an X-ray amount suitable for the lungs is set, the X-ray amount will be insufficient for the liver. In contrast to this, when an X-ray amount suitable for the liver is set, the X-ray amount will be excessive in the lungs.
When the transmitted X-ray amount is insufficient, the S/N (SN ratio) becomes worse because of the reduced amount of X-ray photons detected by the X-ray detector (hereinafter abbreviated as a detector), and the S/N of the entire cross-sectioned image obtained by image reconstruction becomes worse as a result. Conversely, when the transmitted X-ray amount is too large, unnecessary X-rays are applied to the body being examined.
A method for controlling the tube voltage, as disclosed in JP-A-53-110495, and methods for controlling the tube voltage, as disclosed in JP-A-9-108209 and JP-A-10-309271, have been proposed for solving these problems.
However, the method for controlling the tube voltage as described in JP-A-53-110495 has a problem in that, during the scan, the spectrum of the X-rays is changed, since the tube voltage is changed, and thus the CT value cannot be determined. Therefore, a method for controlling the tube current is the main method used at present.
As a method for optimally controlling the tube current in accordance with the characteristics of the body being examined, there is a method of controlling the tube current based on data indicating the amount of X-rays that have passed through the object in the first half period of the scanner rotation, as described in JP-A-10-309271, and a method of determining a pattern for controlling the tube current in advance in accordance with the position of the body which is being examined on the basis of scanograms scanned from two different directions, as described in JP-A-9-108209.
However, the method using the data of X-ray passage through the object in the first half of the scanner rotation, as described in JP-A-10-309271, has a problem in that the X-ray data deviates from one cross-section to another, particularly in a helical scan with a large scan pitch. Further, this method cannot cope with an area in which the X-ray absorption characteristics of the body being examined vary greatly e.g. above and below the diaphragm.
In the method for obtaining the scanograms from two different directions, as described in JP-A-9-108209, the unnecessary X-ray exposure to the body being examined is increased by taking the scanogram photograph twice. Accordingly, this method is contrary to its purpose, i.e. reduction of the exposure through tube current control.
The inventors of the present invention have also proposed a method of optimally controlling the tube current in accordance with characteristics of the body being examined, as described in JP-A-2001-276040. The method described is JP-A-2001-276040 relates to the use of an X-ray CT device that is capable of realizing low exposure scanning by not sending unnecessary X-rays to the body being examined. In this method, a model representing the length of the body where the X-ray passes for each rotation angle of a scanner is stored in memory in advance. When the body is to be scanned, the tube current producing the X-rays is set for every rotation angle of the scanner based on this model. The scan measurement is then made and a cross-sectional image is reconstructed.
The method described in JP-A-2001-276040 lays emphasis on the generation of the X-ray passage length model of the body being examined and the setting of the tube current based on the length of the X-ray passage through the body as given by this X-ray passage length model. However, there is no consideration of the actual amount of X-rays being applied to the body by this set tube current or the X-rays received by the internal organs within the body.
In consideration of the above-mentioned problems, an object of the present invention is to provide an X-ray CT device and a data processing method which make it possible to calculate the exposure, inside and outside the body being examined, to tube current controlled according to a variation control pattern that is automatically set, based on the X-ray passage length model of the body being examined, and to allow an operator to reset the variation control pattern of the tube current in consideration of the X-ray exposure of this body being examined.